This invention relates to a process for the preparation of aqueous dispersions based on acrylic acid-n-butyl ester/acrylonitrile/(meth)acrylic acid copolymers which are eminently suitable for use as binders in unpigmented or pigmented aqueous dispersion paints for metals, wood, plastics and mineral substrates.
It is known from German Pat. No. 980,059 that acidic, stable aqueous dispersions of copolymers of copolymerized units of 1-40% by weight of acrylonitrile, 50-95% by weight of an acrylic acid alkyl ester having 3-8 C-atoms in the alcohol component, 3-6% by weight of acrylic acid and optionally up to 20% by weight of other copolymerizable monomers can be produced by the copolymerization of the aforesaid monomers in the presence of radical formers, emulsifiers which are stable in an acidic medium, and optionally protective colloids and molecular weight regulators at temperatures of up to about 90.degree. C. The dispersions can generally be thickened with ammonia and are used as coating compositions for textiles or glass fabrics, as binders for leather coating paints and as adhesives for paper and for synthetic resin or metal foils.
It cannot be gathered from the aforesaid document nor is it in any way suggested that special, selected monomer combinations of acrylonitrile, acrylic acid-n-butyl ester, and acrylic acid or methacrylic acid or a mixture of methacrylic and acrylic acid, in which a certain proportion of the acrylonitrile may be replaced by styrene and in which the monomer combinations may possibly contain small quantities of N-methoxymethyl methacrylamide, can be converted by a critical combination of process steps which are known per se into aqueous copolymer dispersions which are emimently suitable for use as binders for unpigmented or pigmented paints for metals, wood, synthetic resins and mineral substrates, preferably paints of this type which dry at room temperature.
U.S. Pat. No. 2,787,603 discloses inter alia the conversion of a mixture of 30 parts by weight of acrylonitrile, 65 parts by weight of n-butyl acrylate and 5 parts by weight of methacrylic acid into a copolymer dispersion (see Example IV) by emulsion polymerization and the use of the resulting dispersion, after the addition of a phenol formaldehyde resin which can be diluted with water and adjustment of the pH to about 9.4, as an oven-drying, aqueous coating composition for metal wires or ceramic substrates.
The process according to U.S. Pat. No. 2,787,603 (Example IV) can be used to prepare dispersions having a solids content of 35% by weight. The average particle size of these dispersions is below 0.1.mu.. When ammonia is added, however, these dispersions thicken to form tough pastes which are difficult to handle and which, moreover, are incompatible with pigments and therefore unsuitable for use as binders for aqueous pigmented paints. When attempts are made to employ the process according to the said U.S. patent (Examples IV + I) to prepare dispersions having a solids content of more than 40% by weight, for example 42% by weight, the dispersions obtained have a high coagulate content and when the coagulate is sieved off (average particle size of the dispersion freed from coagulate is 0.16.mu.), the addition of ammonia (10 to 25%) to adjust the pH to 8.5 thickens the dispersions to non-fluid pastes so that they are again unsuitable for use as binders for aqueous, pigmented paints (see experiments for comparison with U.S. Pat. No. 2,787,603 in the experimental part of this application).
An object of the present invention is to provide a process which yields aqueous synthetic resin dispersions with a narrow distribution of particle diameters, which preferably have a concentration of 40-50% by weight and are free from coagulate and specks, stable in storage and stable to electrolytes and shearing forces, and which are compatible with solvents and protective colloids, stable to pigments and inert to fillers, finely divided (average particle diameter about 0.1 to 0.15.mu.) and at the same time stable to freezing and thawing.
At the same time, when adjusted to a pH of 7.5 to 9, preferably 8 to 8.5, the aqueous synthetic resin dispersions should be thickened by the addition of aqueous ammonia to just the limit of flow without substantial alteration in the solids content but without the (meth) acrylic acid content falling below 3% by weight, based on the polymer.
In addition, the minimum film-forming temperature of the dispersions, determined according to DIN 53 787 (without the addition of plasticizer and/or solvent) should not be above 30.degree. C. while the main softening range of the copolymers, determined from the maximum of damping in the torsion vibration experiment according to DIN 53 520, should be at the least 18.degree. C. in order to ensure good film formation at very low temperatures and at the same time good blocking resistance at elevated temperatures (up to 80.degree. C.) combined with great hardness and low tendency to soiling.
The unpigmented and pigmented films prepared from the synthetic resin dispersions should dry free from tears and pores and show good adherence to their substrate, good mechanical properties, high gloss and high blocking resistance. In addition, the dried films should be resistant to water, petroleum hydrocarbons and aromatic solvents and should be resistant to weathering.
The pigmented synthetic resin dispersions (paints) should be capable of being applied not only by the usual methods employed for synthetic resin dispersions but also by brush spreading. None of the known synthetic resin dispersions has hitherto been able to fulfil the totality of these requirements.